In an InGaAs/Si avalanche photodetector (APD), Si is used as the multiplication material to provide avalanche gain, while InGaAs is used as the absorption material. High quantum efficiency, high gain-bandwidth product, and low noise for detection of wavelengths between 1.0 micrometer and 1.6 micrometer can be achieved in this way. We present possible design variations and analyze the performance of these APDs. Particular attention is paid to a 10 Gbit/s APD and we design InGaAs/Si APDs with a 3-dB bandwidth larger than 10 GHz and a gain-bandwidth product greater than 400 GHz.© (1997) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.